I'm talking more about power supplies, not the data signals where 1s and 0s come into it - although they are a whole other can of worm - when it comes to high speed interfaces there is a much bigger gap between working all the time and not working at all. As an example, a DDR3 chip might register Vref+160mV as high and Vref-160mV as low. If at the time the clock transitions the input is only Vref+80mV eg because your trace delay is too high, it's anyone guess what happens - it could depend on chip to chip variations, temperature, etc. Going back to power issues, I reiterate the problem is not the steady state DC voltage that you are talking about, which is what the DC resistance of the trace affects and I have no doubt is in spec, the likely problem is transient drops on the supply line as the devices switches. How the supply will respond to these depends on not just the length of the trace but also the presence and spacing to of a ground plane, decoupling capacitors and their placement and value. The reason that this only affects 4GB boards is the larger memory devices have more transistors to switch, and therefore cause greater transients on the power supply lines, pushing them out of spec for a tiny period of time during that transient, enough to cause issues inside the chip. The resistor issue I mentioned was not intended to be an example of how sensitive DDR chips are, just how hard it is to debug them and work out what is software and what is hardware. Hence even I wouldn't be totally certain about the power issue but it seems pretty likely. Maybe you've done this and I've missed it, but I know you can buy special active scope probes for measuring power rail quality. They are expensive, and you would need access to a suitable good - also expensive, but easier to find someone with - oscilloscope but scrapping boards is also expensive... If you do get stuck it would be interesting to see. I saw some simulations have been done but sometimes measurement is the only way, particularly the exact characteristics of the capacitors and RAM supply demands won't be taken into account in a sim.